Recently, requirements of improving crush safety of automobiles have been increased. For example, as a safety method for front crushing, a method in which a front frame is deformed and impact energy is absorbed while the interior of a passenger space has a high deformation resistance and is inhibited from deforming, thereby securing passenger space, has been thought to be effective. Amount of absorbed impact energy in the front frame are proportional to the product of deformation resistance and deformation stroke. If the impact energy is absorbed with a smaller deformation stroke, several advantages, such as improvement in driving performance by shortening of front overhang and reducing the weight of a vehicle body can be obtained. Therefore, recently, strength of a material (generally a steel sheet) for a front frame has been enhanced.
When a steel sheet for a front frame is greatly strengthened, the yield point of the sheet is enhanced, whereby initial resistance, which is generated at the moment when a vehicle body is involved in a crush, is enhanced. Therefore, it is required to ensure sufficient impact energy absorption while the initial resistance is limited to be as low as possible.
In general, when a steel sheet is highly strengthened and a part such as a front frame is compressed in an elongated direction, the shape of buckling of the front frame is unstable, and the deformation occurs not in the form of an accordion, but in a form that is bent. Absorption efficiency of impact energy is decreased if the deformation occurs in the form of bending and absorption of impact energy cannot be expected to increase even though the material is greatly strengthened. It is known that buckling of a highly strengthened steel sheet is unstable since work hardening properties of the steel sheet are reduced due to high strengthening thereof. That is, when a member buckles first in a longitudinal direction and work hardening of the material is high, the deformation propagates from the buckled portion to the circumference thereof, and then another portion buckles. As a result, buckling with an accordion shape is formed. On the other hand, when work hardening of the material is low, deformation may be concentrated at the first buckling portion, and the member may bend. In general, work hardening property is reduced when a steel sheet is greatly strengthened, thereby producing instability in buckling.
In order to solve such problems, shapes of parts may be designed to stably buckle. However, shapes of parts are limited because of the layout and design in an engine compartment, and shapes of parts cannot be realized as a designer would like. In such a situation, if characteristics of a material are optimized to achieve the objects, energy can be absorbed with no problem and the material can be greatly strengthened. Specifically, if a material with high strength, high yield strength, and high work hardening property is used, increase of initial reaction is inhibited and buckling is stable, thereby effectively absorbing impact energy.
A steel sheet having good crush properties for parts in a vehicle body is disclosed in Patent Reference 1. The steel sheet includes austenite, which forms martensite by strain induced transformation and has a work hardening index of 0.6 or more. Patent Reference 2 discloses a production method for a high-strength steel sheet having good ductility, the method includes hot rolling, cold rolling, and annealing steel containing 0.1 to 0.45% of C and 0.5 or more of Si under predetermined conditions, thereby yielding a steel sheet having a tensile strength of 82 to 113 kgf/mm2, and 2500 kgf/mm2 or more of tensile strength multiplied by elongation. Patent Reference 3 discloses a high-strength steel sheet which contains 0.1 to 0.4 wt % of C and increased amount of Mn with decreased amount of Si, and is subjected to annealing two times, and thereby a tensile strength is 811 to 1240 MPa with high ductility of 28000 MPa or more of tensile strength multiplied by elongation.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2001-130444    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 62-182225    Patent Reference 3: Japanese Unexamined Patent Application Publication No. 7-188834